Customize the from address in the write window? Your provider who is issuing these unlimited aliases should allow you to send using them, but do not bet on it. Outlook.com does not. Their alias function only works from within their web site.

Not sure I understand your answer. I can use these aliases from within Thunderbird, but it's a pain to go into Identities and set them up. And there is no real way to organize them, except via an add-on, which is unwieldy given the number of aliases I have.

Your suggestion also introduces the possibility of spam, and that would create an image of Thunderbird as being a spamming tool.

When it comes to spam, does it really make any difference how an email alias is set up in Thunderbird? That really makes no sense. It's simply a "From" and optionally a "Reply-to" address. It shouldn't matter how it's set up. They are going to show up when a message is composed in any case.

"made up' email addresses have no audit trail, ideal for spammers.

Then it's an issue with email aliases, period. Are you saying the practice should be banned?

In fact, the reason why I use them, is that every site I sign up to gets an alias. That way, if I get spammed, I know who the guilty party is. Then I simply filter out the offending alias, and send "feedback" to the spammer.

Customize the from address in the write window?

What is not to understand?

Your provider who is issuing these unlimited aliases should allow you to send using them, but do not bet on it. Outlook.com does not. Their alias function only works from within their web site.

I can set up as many "alias" addresses in outlook as I like. I can't use them to send mail from a mail client. Outlook simply changes the details set for the alias to those of the primary identity on the way out through their SMTP server. I have not tried it for years now, but I understand Google do something similar with gmail.

Plus addressing (which is a standard) is a better approach for your use case I would guess. I am linking to a discussion on it's use on Google, but most mail providers support the standard. https://eit.ces.ncsu.edu/2023/02/gmail-plus-addressing-the-hidden-feature-that-can-help-you-get-more-out-of-your-inbox/

It is largely to support this that the Thunderbird write window has the customize option for the outgoing address.

Hmm, Outlook does it differently, then. I use Runbox, with my own email domain from Njalla. Runbox does allow one to send emails from aliases, without changing the header to the primary identity. As far as addressing goes, those aren't stored anywhere in Thunderbird, so I can't reuse them without consulting a list. I do keep a list of aliases and the sites they belong to, but I would prefer to enter the new alias via the New Message pop-up, and find it stored under the identity manager, rather than opening it every time I need to add a new one. But I understand if you cannot accommodate this.

EDIT: plus addressing does work with Runbox, BTW

Well, I found an addon "CatchAll Bird" that does pretty much what I'm looking for. It's automated even. I sign up at a site with an alias, then when I get a reply, the addon will create a new identity. It will even create a new folder in the inbox, if you want it to. It does have a limitation, though, due to the way Thunderbird processes mail. It can only do this with your primary account, if you have multiple real email accounts. This might change in the future.

Everybody, thanks for taking the time to respond. I'm going to call this problem solved.